Modelling of two-stage anaerobic digestion using the IWA Anaerobic Digestion Model No. 1 (ADM1)

The aim of the study presented was to implement a process model to simulate the dynamic behaviour of a pilot-scale process for anaerobic two-stage digestion of sewage sludge. The model implemented was initiated to support experimental investigations of the anaerobic two-stage digestion process. The model concept implemented in the simulation software package MATLAB/Simulink is a derivative of the IWA Anaerobic Digestion Model No.1 (ADM1) that has been developed by the IWA task group for mathematical modelling of anaerobic processes. In the present study the original model concept has been adapted and applied to replicate a two-stage digestion process. Testing procedures, including balance checks and 'benchmarking' tests were carried out to verify the accuracy of the implementation. These combined measures ensured a faultless model implementation without numerical inconsistencies. Parameters for both, the thermophilic and the mesophilic process stage, have been estimated successfully using data from lab-scale experiments described in literature. Due to the high number of parameters in the structured model, it was necessary to develop a customised procedure that limited the range of parameters to be estimated. The accuracy of the optimised parameter sets has been assessed against experimental data from pilot-scale experiments. Under these conditions, the model predicted reasonably well the dynamic behaviour of a two-stage digestion process in pilot scale.     

Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge

The performance of thermophilic and mesophilic temperature co-phase anaerobic digestions for sewage sludge, using the exchange process of the digesting sludge between spatially separated mesophilic and thermophilic digesters, was examined, and compared to single-stage mesophilic and thermophilic anaerobic digestions. The reduction of volatile solids from the temperature co-phase anaerobic digestion system was dependent on the sludge exchange rate, but was 50.7-58.8%, which was much higher than 46.8% of single-stage thermophilic digestion, as well as 43.5% of the mesophilic digestion. The specific methane yield was 424-468 mL CH(4) per gram volatile solids removed, which was as good as that of single-stage mesophilic anaerobic digestion. The process stability and the effluent quality in terms of volatile fatty acids and soluble chemical oxygen demand of the temperature co-phase anaerobic digestion system were considerably better than those of the single-stage mesophilic anaerobic processes. The destruction of total coliform in the temperature co-phase system was 98.5-99.6%, which was similar to the single-stage thermophilic digestion. The higher performances on the volatile solid and pathogen reduction, and stable operation of the temperature co-phase anaerobic system might be attributable to the well-functioned thermophilic digester, sharing nutrients and intermediates for anaerobic microorganisms, and selection of higher substrate affinity anaerobic microorganisms in the co-phase system, as a result of the sludge exchange between the mesophilic and thermophilic digesters.     

Fate of pharmaceutical and personal care products (PPCPs) during anaerobic digestion of sewage sludge

The behaviour of 13 pharmaceutical and personal care products (PPCPs) has been studied during anaerobic digestion of sewage sludge: two musks (Galaxolide and Tonalide), one tranquilliser (Diazepam), one anti-epileptic (Carbamazepine), three anti-phlogistics (Ibuprofen, Naproxen and Diclofenac), two antibiotics (Sulfamethoxazole and Roxithromycin), one X-ray contrast medium (Iopromide) and three oestrogens (Estrone, 17beta-oestradiol and 17alpha-ethinyloestradiol). Two parallel processes have been carried out, one in mesophilic range (37 degrees C) and the other in thermophilic range (55 degrees C). The influence of temperature and sludge retention time (SRT) has been analysed. Among the substances considered, the higher removal efficiencies were achieved for the antibiotics, natural oestrogens, musks and Naproxen. For the other compounds, the values ranged between 20% and 60%, except for Carbamazepine, which showed no elimination. The removal of oestrogens, Diazepam and Diclofenac occurred after sludge adaptation. In general, no influence of SRT and temperature on PPCPs removal was observed. Considering the difficulty of obtaining reliable PPCPs concentrations, especially those corresponding to the fractions sorbed onto sludge, a methodology to validate the experimental data has been developed and successfully applied.     

Effect of linear alkylbenzene sulphonates (LAS) on the anaerobic digestion of sewage sludge

Batch anaerobic biodegradation tests with different alkylbenzene sulphonates (LAS) at increasing concentrations were performed in order to investigate the effect of LAS homologues on the anaerobic digestion process of sewage sludge. Addition of LAS homologues to the anaerobic digesters increased the biogas production at surfactant concentrations 5-10 g/kg dry sludge and gave rise to a partial or total inhibition of the methanogenic activity at higher surfactant loads. Therefore, at the usual LAS concentration range in sewage sludge, no adverse effects on the anaerobic digesters functioning of a wastewater treatment plant (WWTP) can be expected. The increase of biogas production at low surfactant concentrations was attributed to an increase of the bioavailability and subsequent biodegradation of organic pollutants associated with the sludge, promoted by the surfactant adsorption at the solid/liquid interface. When the available surfactant fraction in the aqueous phase instead of the nominal surfactant concentration was used to evaluate the toxicity of LAS homologues, a highly significant relationship between toxicity and alkyl chain length was obtained. Taking into account the homologue distribution of commercial LAS in the liquid phase of the anaerobic digesters of a WWTP, an EC(50) value of 14 mg/L can be considered for LAS toxicity on the anaerobic microorganisms.     

A waste characterisation procedure for ADM1 implementation based on degradation kinetics

In this study, a procedure accounting for degradation kinetics was developed to split the total COD of a substrate into each input state variable required for Anaerobic Digestion Model n°1. The procedure is based on the combination of batch experimental degradation tests (“anaerobic respirometry”) and numerical interpretation of the results obtained (optimisation of the ADM1 input state variable set). The effects of the main operating parameters, such as the substrate to inoculum ratio in batch experiments and the origin of the inoculum, were investigated. Combined with biochemical fractionation of the total COD of substrates, this method enabled determination of an ADM1-consistent input state variable set for each substrate with affordable identifiability. The substrate to inoculum ratio in the batch experiments and the origin of the inoculum influenced input state variables. However, based on results modelled for a CSTR fed with the substrate concerned, these effects were not significant. Indeed, if the optimal ranges of these operational parameters are respected, uncertainty in COD fractionation is mainly limited to temporal variability of the properties of the substrates. As the method is based on kinetics and is easy to implement for a wide range of substrates, it is a very promising way to numerically predict the effect of design parameters on the efficiency of an anaerobic CSTR. This method thus promotes the use of modelling for the design and optimisation of anaerobic processes.     

Occurrence of synthetic musks in Korean sewage sludges

Six polycyclic musks (PCMs) and five nitro musks (NMs) were analyzed in dewatered sludge samples from 13 municipal, 4 livestock and 1 pharmaceutical wastewater treatment plants (WWTPs) in South Korea. 1,3,4,6,7,8-Hexahydro-4,6,6,7,8,8-hexamethylcyclopenta-(g)-2-benzopyran (HHCB; Galaxolide^®, Abbalide^®), and 7-acetyl-1,1,3,4,4,6-hexamethyl-1,2,3,4-tetrahydronaphthalene (AHTN; Tonalide^®, Fixolide^®) were the two predominant PCMs in the sludge samples, at concentrations ranging from 0.52 to 82.0 mg/kg dry weight (dw) and 0.12 to 28.8 mg/kg (dw), respectively, suggesting the extensive use of these two PCMs in South Korea. Hierarchical cluster analysis (HCA) revealed the different usage patterns of PCMs between livestock cultivating farm and household. The levels of HHCB and AHTN in the effluent discharged from the WWTPs were lower than the threshold effect levels derived for fish, suggesting low potential risks to aquatic organisms. Nevertheless, the calculated discharge of synthetic musks (SMs) from sludge in each target WWTP ranged from 36 to 10,961 g/d. These results indicate the necessity of conducting monitoring studies in marine ecosystems due to ocean disposal of sludge, particularly focused on the two prevailing PCMs.     

Multidimensional modelling to investigate interspecies hydrogen transfer in anaerobic biofilms

Anaerobic digestion is a multistep process, mediated by a functionally and phylogenetically diverse microbial population. One of the crucial steps is oxidation of organic acids, with electron transfer via hydrogen or formate from acetogenic bacteria to methanogens. This syntrophic microbiological process is strongly restricted by a thermodynamic limitation on the allowable hydrogen or formate concentration. In order to study this process in more detail, we developed an individual-based biofilm model which enables to describe the processes at a microbial resolution. The biochemical model is the ADM1, implemented in a multidimensional domain. With this model, we evaluated three important issues for the syntrophic relationship: (i) Is there a fundamental difference in using hydrogen or formate as electron carrier? (ii) Does a thermodynamic-based inhibition function produced substantially different results from an empirical function? and; (iii) Does the physical co-location of acetogens and methanogens follow directly from a general model. Hydrogen or formate as electron carrier had no substantial impact on model results. Standard inhibition functions or thermodynamic inhibition function gave similar results at larger substrate field grid sizes (> 10 microm), but at smaller grid sizes, the thermodynamic-based function reduced the number of cells with long interspecies distances (> 2.5 microm). Therefore, a very fine grid resolution is needed to reflect differences between the thermodynamic function, and a more generic inhibition form. The co-location of syntrophic bacteria was well predicted without a need to assume a microbiological based mechanism (e.g., through chemotaxis) of biofilm formation.     

Isolation, identification and utilization of thermophilic strains in aerobic digestion of sewage sludge

Two representative thermophilic bacterial strains (T1 and T2) were isolated from a one-stage autothermal thermophilic aerobic digestion pilot-scale reactor. 16S rRNA gene analysis indicated that they were Hydrogenophilaceae and Xanthomonodaceae. These isolated strains were inoculated separately and/or jointly in sewage sludge, to investigate their effects on sludge stabilization under thermophilic aerobic digestion condition. Four digestion conditions were tested for 480 h. Digestion without inoculation and inoculation with strain T2, as well as joint- inoculation with strains T1 and T2, achieved 32.6%, 43.0%, and 38.2% volatile solids (VS) removal, respectively. Removal in a digester inoculated with stain T1 only reached 27.2%. For the first 144 h, the three inoculated digesters all experienced higher VS removal than the digester without inoculations. Both specific thermophilic strains and micro-environment significantly affected the VS removal. DGGE profiles revealed that the isolated strains T1 and T2 can successfully establish in the thermophilic digesters. Other viable bacteria (including anaerobic or facultative microbes) also appeared in the digestion system, enhancing the microbial activity.     

Performance parameter prediction for sewage sludge digesters using reflectance FT-NIR spectroscopy

This study investigates the use of Fourier transform near infrared (FT-NIR) spectroscopy combined with principle components analysis (PCA) and partial least squares regression (PLS-R) as part of a possible process monitoring system for sewage sludge anaerobic digesters. The ability of FT-NIR with PCA to distinguish between different stages of the AD process was investigated, it was found that waste activated sludge (WAS), primary, feed (Primary:WAS 70:30) and digested sludge were distinguishable from each other using this technique. PLS-R was used successfully to track differing proportions of primary:WAS in feedstocks of 5% total solids (Coefficient of Efficiency (CE) = 0.93). The study also looked at the ability of reflectance mode NIR spectroscopy to track process parameters important for stability. Temperature and organic loading rate variations were employed to stress the digesters. Predictive models were produced for volatile fatty acids (VFA), bicarbonate alkalinity (BA) and total and volatile solids (TS and VS) and independently validated for each digester. The models were able to track the relevant process parameters: TS (CE = 0.75), VS (CE = 0.75), BA (CE = 0.71), and VFA (CE = 0.69). This technique could be used to improve the performance of sewage sludge anaerobic digesters.     

Optimization of the hydrolytic-acidogenic anaerobic digestion stage (55 degrees C) of sewage sludge: influence of pH and solid content

In conventional single-stage anaerobic digestion processes, hydrolysis is regarded as the rate-limiting step in the degradation of complex organic compounds, such as sewage sludge. Two-stage systems have been proposed to enhance this process. However, so far it is not clear which are the best conditions for a two-stage anaerobic digestion process of sewage sludge, in terms of temperature and hydraulic retention time of each stage. The aim of this work was to determine the optimal conditions for the hydrolytic-acidogenic stage treating real sludge with a high concentration of total solids (40-50gL(-1)) and volatile solids (25-30gL(-1)), named high concentration sludge. The variables considered for this first stage were: hydraulic retention time (1-4 days) and temperature (55 and 65 degrees C). Maximum volatile fatty acids generation was obtained at 4 days and 3 days hydraulic retention time for 55 degrees C and 65 degrees C, respectively. Consequently, 4 days hydraulic retention time and temperature of 55 degrees C were set as the working conditions for the hydrolytic-acidogenic stage treating high concentration sludge. The results obtained when operating with high concentration sludge were compared with a low concentration sludge consisting of 17-28gL(-1) total solids and 13-21gL(-1) volatile solids. The effect of decreasing the influent sludge pH, when working at the optimal conditions established, was also evaluated.     

Acidogenic fermentation of proteinaceous sewage sludge: Effect of pH

Proteinaceous sewage sludge represents the sludge in which the protein content exceeds the carbohydrate content, accounts for a considerable part of sewage sludge. In this study, volatile fatty acids (VFA) production and synthesis pathway as well as bacterial community in acidogenic fermentation of proteinaceous sewage sludge under different pH conditions were investigated. The results indicated that the alkaline pH improved the solubilization and biodegradation of proteins in the sludge. The soluble protein concentration at pH 11.0 was 67.88% higher than that at pH 3.0. Its biodegradation efficiencies were reduced with a decrease in pH. The pH influenced not only the total VFA production yield, but also the percentage of individual VFA. The total VFA yield at pH 9.0 was 10.70 times of that at pH 3.0. The metabolic conversion pathway of proteins to VFA in acidogenic fermentation systems was investigated using a stoichiometry approach. The Stickland reaction was found to be the main pathway for the VFA accumulation from the decomposition of proteins. pH also significantly influenced the biodiversity and bacterial community in the system. The abundance of microorganisms under alkaline or acidic pH conditions was less than that under neutral pH condition, suggesting that most anaerobic fermentative microorganisms could not tolerate the hostile environment. The terminal restriction fragment length polymorphism analysis showed that, when the pH was reduced from 12.0 to 7.0 and finally to 3.0, the dominant bacterial genuses and percentage of the total microorganisms were Granulicatella genus (65%), Clostridium genus (28%) and Bacillus genus (71%), respectively.     

Anaerobic digestion of tannery soak liquor with an aerobic post-treatment

The leather industry occupies a place of prominence in the Indian economy due to its massive potential for employment, growth and exports. The potential environmental impact of tanning is significant. This study focuses on tannery soak liquor, generated by the soaking of hides and skins, which is characterised by high organic load and high salinity. For these reasons, the soak liquor should be segregated and pre-treated separately before being mixed with the composite wastewater, made of all other streams mixed together. The anaerobic digestion of tannery soak liquor was studied using a UASB. COD removal reached 78% at an OLR of 0.5 kg COD m(-3) d(-1), a HRT of 5 days and a TDS concentration of 71 gl(-1). The combination of the UASB with an aerobic post-treatment enhanced the performance of the overall wastewater treatment process and the COD removal efficiency of the combined anaerobic/aerobic treatment system reached 96%. However, for effective operation, the system had to be operated at very low OLRs, which affects the economic viability of such a process.     

Seasonal and time variability of heavy metal content and of its chemical forms in sewage sludges from different wastewater treatment plants

Sewage sludges obtained from seven wastewater treatment plants from the province of Salamanca, Spain, were periodically sampled to determine seasonal and time variation of their elemental composition over 2000 to 2002. The aim of this paper was to provide additional insight to evaluate the potential environmental impact following soil incorporation of these materials as amendments. Aqua regia extractable metals (pseudo total content) of Cd, Cr, Cu, Ni, Pb and Zn were determined and furthermore, the main chemical forms of metals within the sludge were evaluated using a five-step fractionation procedure. All the studied sludges displayed high fertility properties due to their richness of OC, P and K. Total mean concentrations of Cd, Cr, Cu, Ni, Pb and Zn in the sludges were within the regulation of the Spanish legislation. Using an multifactor analysis of variance, significant differences between Cr, Cu, Ni, Pb and Zn pseudo total contents (p<0.01) of sludges at different sites were found while the Cd content was statistically similar. Also significant differences were found between these pseudo total contents of heavy metals in samples collected along the time after three years (0.001相似文献   

Mesophilic and thermophilic anaerobic digestion of primary and secondary sludge. Effect of pre-treatment at elevated temperature

Anaerobic digestion is an appropriate technique for the treatment of sludge before final disposal and it is employed worldwide as the oldest and most important process for sludge stabilization. In general, mesophilic anaerobic digestion of sewage sludge is more widely used compared to thermophilic digestion. Furthermore, thermal pre-treatment is suitable for the improvement of stabilization, enhancement of dewatering of the sludge, reduction of the numbers of pathogens and could be realized at relatively low cost especially at low temperatures. The present study investigates (a) the differences between mesophilic and thermophilic anaerobic digestion of sludge and (b) the effect of the pre-treatment at 70 degrees C on mesophilic and thermophilic anaerobic digestion of primary and secondary sludge. The pre-treatment step showed very positive effect on the methane potential and production rate upon subsequent thermophilic digestion of primary sludge. The methane production rate was mostly influenced by the pre-treatment of secondary sludge followed by mesophilic and thermophilic digestion whereas the methane potential only was positively influenced when mesophilic digestion followed. Our results suggest that the selection of the pre-treatment duration as well as the temperature of the subsequent anaerobic step for sludge stabilization should depend on the ratio of primary to secondary sludge.     

Anaerobic sewage treatment in a one-stage UASB reactor and a combined UASB-Digester system

The treatment of sewage at 15 degrees C was investigated in a one-stage upflow anaerobic sludge blanket (UASB) reactor and a UASB-Digester system. The latter consists of a UASB reactor complemented with a digester for mutual sewage treatment and sludge stabilisation. The UASB reactor was operated at a hydraulic retention time of 6h and a controlled temperature of 15 degrees C, the average sewage temperature during wintertime of some Middle East countries. The digester was operated at 35 degrees C. The UASB-Digester system provided significantly (significance level 5%) higher COD removal efficiencies than the one-stage UASB reactor. The achieved removal efficiencies in the UASB-Digester system and the one-stage UASB reactor for total, suspended, colloidal and dissolved COD were 66%, 87%, 44% and 30%, and 44%, 73%, 3% and 5% for both systems, respectively. The stability values of the wasted sludge from the one-stage UASB reactor and the UASB-Digester system were, respectively, 0.47 and 0.36g CH(4)-COD/g COD. Therefore, the anaerobic sewage treatment at low temperature in a UASB-Digester system is promising.     

Modeling microbial diversity in anaerobic digestion through an extended ADM1 model

The anaerobic digestion process comprises a whole network of sequential and parallel reactions, of both biochemical and physicochemical nature. Mathematical models, aiming at understanding and optimization of the anaerobic digestion process, describe these reactions in a structured way, the IWA Anaerobic Digestion Model No. 1 (ADM1) being the most well established example. While these models distinguish between different microorganisms involved in different reactions, to our knowledge they all neglect species diversity between organisms with the same function, i.e. performing the same reaction. Nevertheless, available experimental evidence suggests that the structure and properties of a microbial community may be influenced by process operation and on their turn also determine the reactor functioning. In order to adequately describe these phenomena, mathematical models need to consider the underlying microbial diversity. This is demonstrated in this contribution by extending the ADM1 to describe microbial diversity between organisms of the same functional group. The resulting model has been compared with the traditional ADM1 in describing experimental data of a pilot-scale hybrid Upflow Anaerobic Sludge Filter Bed (UASFB) reactor, as well as in a more detailed simulation study. The presented model is further shown useful in assessing the relationship between reactor performance and microbial community structure in mesophilic CSTRs seeded with slaughterhouse wastewater when facing increasing levels of ammonia.     

Long-term effect of ZnO nanoparticles on waste activated sludge anaerobic digestion

The increasing use of zinc oxide nanoparticles (ZnO NPs) raises concerns about their environmental impacts, but the potential effect of ZnO NPs on sludge anaerobic digestion remains unknown. In this paper, long-term exposure experiments were carried out to investigate the influence of ZnO NPs on methane production during waste activated sludge (WAS) anaerobic digestion. The presence of 1 mg/g-TSS of ZnO NPs did not affect methane production, but 30 and 150 mg/g-TSS of ZnO NPs induced 18.3% and 75.1% of inhibition respectively, which showed that the impact of ZnO NPs on methane production was dosage dependant. Then, the mechanisms of ZnO NPs affecting sludge anaerobic digestion were investigated. It was found that the toxic effect of ZnO NPs on methane production was mainly due to the release of Zn²⁺ from ZnO NPs, which may cause the inhibitory effects on the hydrolysis and methanation steps of sludge anaerobic digestion. Further investigations with enzyme and fluorescence in situ hybridization (FISH) assays indicated that higher concentration of ZnO NPs decreased the activities of protease and coenzyme F₄₂₀, and the abundance of methanogenesis Archaea.     

Anaerobic biodegradability and treatment of grey water in upflow anaerobic sludge blanket (UASB) reactor

Feasibility of grey water treatment in an upflow anaerobic sludge blanket (UASB) reactor operated at different hydraulic retention time (HRT) of 16, 10 and 6h and controlled temperature of 30 degrees C was investigated. Moreover, the maximum anaerobic biodegradability without inoculum addition and maximum removal of chemical oxygen demand (COD) fractions in grey water were determined in batch experiments. High values of maximum anaerobic biodegradability (76%) and maximum COD removal in the UASB reactor (84%) were achieved. The results showed that the colloidal COD had the highest maximum anaerobic biodegradability (86%) and the suspended and dissolved COD had similar maximum anaerobic biodegradability of 70%. Furthermore, the results of the UASB reactor demonstrated that a total COD removal of 52-64% was obtained at HRT between 6 and 16 h. The UASB reactor removed 22-30% and 15-21% of total nitrogen and total phosphorous in the grey water, respectively, mainly due to the removal of particulate nutrients. The characteristics of the sludge in the UASB reactor confirmed that the reactor had a stable performance. The minimum sludge residence time and the maximum specific methanogenic activity of the sludge ranged between 27 and 93 days and 0.18 and 0.28 kg COD/(kg VS d).     

Performance improvement and decolourization in the high-solid thermophilic anaerobic digestion of thermally pretreated sewage sludge

This laboratory-scale study attempted performance improvement and decolourization in the high-solid thermophilic anaerobic digestion of thermally pretreated sewage sludge, as it tends to be disturbed by ammonia inhibition and colour generation. Sewage sludge was adjusted to 7%–8% total solids (TS), and pretreated at 150°C for 1 h. The digesters were operated at 55°C and 20 days hydraulic retention time. An addition of powdered activated carbon (approximately 2% of the feed TS) significantly contributed to the removal of propionate and reduced the colour in digested sludge by about 27%. Microbial analysis detected less abundance of bacterial Synergistia and archaeal Methanosarcina and implied more hydrogenotrophic methanogenesis with the activated carbon addition. Conditioning with ferric chloride for dewatering digested sludge mitigated the colour of dewatered liquor by about 67%. Therefore, these methods were demonstrated to be effective and partly overcome the above-mentioned problems.     

Anaerobic stabilisation and conversion of biopolymers in primary sludge--effect of temperature and sludge retention time

The effect of sludge retention time (SRT) and process temperature on the hydrolysis, acidification and methanogenesis of primary sludge was investigated in completely stirred tank reactors (CSTRs). The CSTRs were operated to maintain SRTs of 10, 15, 20 and 30 days at process temperatures of 25 degrees C and 35 degrees C. The rates of hydrolysis and the biodegradability of primary sludge were assessed in batch reactors incubated at 15 degrees C, 25 degrees C and 35 degrees C. The results revealed that the major amount of sludge stabilisation occurred between 0 and 10 days at 35 degrees C and 10 and 15 days at 25 degrees C. Hydrolysis was found to be the rate limiting-step of the overall digestion process, for the reactors operated at 35 degrees C and 25 degrees C, except for the reactor operated at 10 days and 25 degrees C. At the latter conditions, methanogenesis was the rate-limiting step of the overall digestion process. Proteins hydrolysis was limited to a maximum value of 39% at 30 days and 35 degrees C due to proteins availability in the form of biomass. The biodegradability of primary sludge was around 60%, and showed no temperature dependency. The hydrolysis of the main biopolymers and overall particulate COD of the primary sludge digested in CSTRs were well described by first-order kinetics, in case hydrolysis was the rate-limiting step. Similarly, the hydrolysis of the overall particulate COD of the primary sludge digested in batch reactors were described by first-order kinetics and revealed strong temperature dependency, which follows Arrhenius equation.